1. Field of the Invention
This invention relates generally to treads having a configuration and/or properties for improving or preventing irregular wear on the tread of a tire, and, more specifically, to a tire that has a tread with a gradient of properties that helps to prevent or improve irregular wear, especially suitable for tires that are free rolling such as steer or trailer tires used on trucks.
2. Description of the Related Art
The appearance of abnormal wear is a frequent cause for removing tires, especially truck tires used as steer or trailer tires, because they are not driven and are therefore prone to develop abnormal wear, especially in the shoulder of the tires. There are many attempted solutions to this problem that involve the use of different tread materials that attempt to prevent or improve the abnormal wear characteristics of the tire.
While it is known that x, y and z stresses placed on tread elements play a role in the development of irregular wear, the modification of x-stresses is often focused on in order to improve the irregular wear performance of a tire. One example of this is U.S. Pat. No. 6,247,512. It discloses several combinations of tread compounds that have different modulus and hysteresis that alternate in the tread. This patent, as shown by FIG. 1, theorizes that hysteretic compounds improve abnormal wear performance by changing the x-stresses or circumferential stresses of tread elements, such as ribs or blocks. By using different materials with different properties, the negative kick-out stresses 20 are ameliorated or even changed to positive stresses 22, helping to prevent irregular wear. However, there is no unified approach or theory given by this patent for designing a tread for maximizing this benefit. For example, there are multiple embodiments that switch a first tread material having greater hysteresis and lower modulus with a second material having lower hysteresis and higher modulus so that one is on top of the other in one embodiment while the other is on top in yet a further embodiment. Accordingly, the reason why these various configurations work is not fully developed and a further refinement or improvement is warranted.
Accordingly, it is desirable to find a unified theory and design approach that optimizes the use of tread compounds having different properties to minimize the appearance of abnormal wear in a tire so that tires with treads having even better wear performances can be obtained that also have desirable rolling resistance and heat generation characteristics as well.